1. Field of the Invention
The present invention relates to aqueous binder compositions containing water-dispersible or water-soluble, hydroxyl group-containing, polymer-modified polyester resins based on olefinically unsaturated polyester resins and at least one heat-activatable cross-linking resin; and the use of these binder compositions in heat curing coating compositions.
2. Description of the Prior Art
Modern aqueous binders are able to be substituted in many applications for organically dissolved binders. In some applications, e.g, in industrial or automobile coating, both the coating compositions when ready for use and the resulting coatings do not satisfy the exacting requirements. These applications demand good flow and good optical film properties, such as high gloss. High resistance to chemicals and solvents or premium motor fuel, good resistance to condensed water and a low tendency towards yellowing, especially during overstoving, are other necessary properties in addition to the good optical properties. Also, aqueous binders should contain as low a level as possible of organic solvents in order also to comply with more stringent environmental requirements.
DE-A-2,824,418 describes specific carboxyl group-containing polyesters having an acid value of from 30 to 150 which are able to be cross-linked thermally with amine resins, phenolic resins and/or blocked polyisocyanates. A disadvantage of these resins for conventional use is the need to utilize alcoholic or glycolic solvents in order to produce stable solutions of the high molecular weight polyester. Another disadvantage is the occurrence of molecular degradation caused by saponification when the dispersions, which are prepared in water followed by neutralization with amines, are stored for any length of time. Without the addition of organic co-solvents, they show a high "water hump" which gives rise to low solids lacquers.
DE-A-3,213,160 describes binders based on mixtures of acid-functional polyesters and acid-functional acrylic copolymers. In this instance the acrylic copolymers are either prepared separately in solution or as an emulsion copolymer. The polyester portion of these dispersions is also vulnerable to saponification-induced molecular degradation.
EP-A-0,391,271 describes stoving lacquers based on polyester oligomer polyacrylate resins, in which the resins are prepared in organic solution, mixed with amino resin cross-linking agents and subsequently dispersed in water. The examples describe the preparation of the resins by the polymerization of the polyacrylate portion in the presence of specific polyester oligomers, which are not radically polymerizable and have an average molecular weight (according to T. C. Patton, Alkyd Resin Technology "Formulating Techniques and Allied Calculations", 1962, pp. 106 et seq.) of 200 to 1000, an OH number of 280 to 600 and an acid number of from 0 to 1.5.
DE-A4,223,182 describes polyester oligomer-polyacrylate dispersions which have the same specifications as in DE-A-3,910,829 and are utilized with a special cross-linking agent mixture of amino resins and blocked polyisocyanates to provide stoving coatings having improved resistance to sulphuric acid.
The possibility of problems arising with regard to stability in storage in the case of polyester-polyacrylate dispersions in which the polyester part and the polyacrylate part are not coupled together at least partially by chemical means is well known. Such coating compositions also contain considerable quantities of organic co-solvents.
DE-A-4,137,896 describes aqueous binder compositions based on aqueous solutions or dispersions of organic polyhydroxyl compounds and polyisocyanates having free isocyanate groups, wherein polyester resins "grafted" with vinyl monomers and containing carboxylate groups and hydroxyl groups are utilized as polyhydroxyl compounds. These polyester resins "grafted" with vinyl monomers are polymer-modified polyester resins which have been prepared by copolymerizing olefinically unsaturated monomers in the presence of olefinically unsaturated polyester resins. The resulting polymers are the same as those used according to the present invention. However, this prior publication, which is solely directed to two-component polyurethane coating compositions that cure at room temperature, does not teach or suggest the necessity these polymer-modified polyester resins in place of the polyester resins described in the previous references in order to provide high-grade stoving lacquers which result in lacquer coatings having improved resistance to yellowing during overstoving.
It has now surprisingly been found that compositions containing polymer-modified polyester resins in combination with known heat-activatable cross-linking resins represent water-soluble or water-dispersible binder compositions which even when overstoved result in coatings that exhibit a particularly low tendency towards yellowing and also combine good optical film properties with good resistance to chemicals, solvents and water.
These binder compositions are eminently compatible not only with water but also with the other lacquer components, have a low volatile organics content, and may be processed to provide high-grade aqueous coating and sealing compositions which are stable in storage.